Pressure sensors have become ubiquitous the past few years as they have found their way into several new types of products. Utilized in automotive, industrial, consumer, and medical products the demand for pressure sensors has skyrocketed and shows no signs of abating.
Pressure sensors typically include a diaphragm supported by a frame. When a pressure sensor experiences a pressure, the diaphragm responds by changing shape. This causes one or more characteristics of electronic components on the diaphragm to change. These changing characteristics can be measured, and from this the pressure can be determined.
Often, the electronic components are resistors that are configured as a Wheatstone bridge located on the diaphragm. As the diaphragm distorts due to a pressure, the resistance of the resistors change. This change results in an output of the Wheatstone bridge, from which the pressure can be measured.
Unfortunately, other factors can cause characteristics of these components to change. For example, various layers or other components may cause stress to be applied to components, such as resistors, thereby changing their values. Also, various layers used in manufacturing a pressure sensor may have coefficients of thermal expansion that are different from silicon. As a pressure sensor changes temperature, these layers may provide different stresses to the components. For example, one or more of these layers may provide stresses to resistors in a Wheatstone bridge. These stresses may change their resistances, and/or cause the diaphragm to change shape, thereby causing an output of the Wheatstone bridge even in the absence of an applied pressure. This output signal in the absence of an input pressure may be referred to as an offset. In addition, the offset can change as a function of temperature due to the change in stress over temperature which is referred to as the temperature coefficient of offset.
The offset and other mismatches can cause errors, skew pressure readings, and limit the usefulness of a pressure sensor. As a result expensive external compensation would be required to minimize the errors due to the offset and the temperature coefficient of offset.
Thus, what are needed are circuits, methods, and apparatus that provide pressure sensors having components with reduced variations due to stresses caused by various layers and components that are included in the manufacturing process.